1. Field of the Invention
This invention relates to a finisher for an image forming apparatus, for example a copy machine, printer or similar image forming apparatus, and particularly to a sheet transport system which simultaneously transports plural sheets from an image forming apparatus to an image formed sheet stacker by way of a finisher.
2. Discussion of the Background Art
In a background finisher of an image forming apparatus, the finisher receives and stacks image formed sheets from an image forming apparatus and then carries out various finishing processes on the image formed sheets. As examples of the finishing processes, the finisher can staple, make holes, fold, stamp the image formed sheets, etc.
In a case that a finisher is combined with an automatic document feeder (ADF) in an image forming apparatus, for example, the finisher may receive a second set of image formed sheets after a first set of image formed sheets is stapled and transported to the image formed sheet stacker. However, in this case the total finishing operation time becomes longer than the image forming process time because the finisher system requires both a finishing operation time, such as a time for stapling, and a transporting time for transporting sheets from the finisher to the image formed sheet stacker.
Therefore, as such a finishing time is longer than an image forming time, the image forming operation may be suspended until the finishing operation ends. When the image forming operation is thus suspended, the productivity of the image forming apparatus decreases.
Some attempts to solve this problem involve simultaneously transporting piled plural image formed sheets to a finisher. Specifically, a finisher system may include a plural sheet transport path between an image formed sheet receiver and a finisher. The finisher system can then temporally suspend a part of a set of image formed sheets which are to be finished in the above plural sheet transport path.
One such system is disclosed in Japanese Laid Open Patent Application 5-286619 and as shown in FIGS. 1 and 2. FIG. 1 is a cross-sectional view of this background sheet path system with finisher in which a finisher system 313 includes a first sheet transport path 312 and a second sheet transport path 311 which is longer than the first sheet transport path 312; these sheet transport paths 311, 312 are also formed between two pairs of rollers 307 and 305. The first sheet transport path 312 includes a pair of rollers 316 and the second sheet transport path 311 includes a pair of rollers 315. These sheet transport paths 311 and 312 branch off after the pair of rollers 305 at an entrance and merge with each other at the pair of rollers 307 at an exit. Furthermore, a path selector 314 is set up at the junction point between the first sheet transport path 312 and the second sheet transport path 311.
Referring to FIG. 2(a), the finisher system inserts a first sheet 310a to the pair of rollers 305 through the sheet guide 308 from an image forming apparatus. The path selector 314 selects the second sheet transport path 311. The inserted first sheet 310a is thereby transported to the second sheet transport path 311 via the path selector 314 by rotation of the pair of rollers 305.
Referring to FIG. 2(b), while the inserted first sheet 310a is passing through the second sheet transport path 311, the path selector 314 shifts its position to select the first sheet transport path 312. The pair of rollers 305 then guides a second sheet 310b to the first sheet transport path 312.
Referring to FIG. 2(c), both sheets 310a and 310b are joined and piled at the entrance to the pair of feeding rollers 307 along each sheet transport path 311 and 312 because each path has a different distance. Simultaneously, the path selector 314 shifts its position to again select the second sheet transport path 311. A third sheet 310c is then fed to the second sheet transport path 311 by way of the path selector 314 by rotation of the pair of rollers 305.
Referring to FIG. 2(d), the first sheet 310a and the second sheet 310b are transported at this time. Furthermore, the path selector 314 then shifts its position to select the first sheet transport path 312. A fourth sheet 310d is then fed to the first sheet transport path 312 by way of the path selector 314 by rotation of the pair of rollers 305 while the third sheet 310c is transported in the second sheet transport path 311.
Now referring to FIG. 3, another background art system is disclosed in Japanese Laid Open patent application 5-147372, U.S. Pat. No. 5,344,130. In this background art, a finisher 440 receives a first sheet S1 and transports the first sheet S1 along a main path 441c during a finishing operation for a first set of sheets S on a finishing plate 446, for example to be stapled by a stapler 449, as shown in FIG. 3(a). Then, the finisher 440 suspends rotating of a pair of rollers 443 during the finishing operation for the first set of sheets. As a result of suspending rotating of the pair of rollers 443, the sheet S1 is also suspended at the pair of rollers 443.
Referring to FIG. 3(b), a second sheet S2 is transported along a bypath 441b by shifting a deflector 444. As a result, the finisher 440 is capable of maintaining a time of a finishing time for the first set of sheets S. Sequentially, after the finisher 440 completes the finishing operation for the first set of sheets, the pair of rollers 442 and 443 is synchronously rotated. Finally, sheets S1 and S2 are simultaneously fed from main path 441c and bypath 441b to the finishing plate 446 as shown in FIG. 3(c).
The inventor of the present invention has identified that the above background art systems have the following problems. In the background art system of FIGS. 1 and 2, when the finisher system transports the both sheets 310a, 310b, these sheets may reach roller pair 307 with a shifted position (i.e., the first shift 310a may be shifted upstream and reach roller pair 307 prior to second sheet 310b) in a transporting direction toward the sheet transport paths. As a result, the both sheets 310a, 310b are joined and piled with a shifted position in the transporting direction at roller pair 307. This sheet transportation with the shifted position may cause of a sheet jam and improper stacking at a finisher. The finisher then may not be capable of carrying out a proper finishing operation.
Furthermore, in the background art system of FIG. 3 the sheets ejected from the pair of rollers 442 and 443 fall on account of their own weights. Therefore, when the finisher system transports the both sheets S1, S2 with a shifted position in a transporting direction toward the sheet transport paths, the finisher may not be capable of carrying out a proper finishing operation. The both sheets S1, S2 are joined and piled at the pair of rollers 442 and 443. Therefore, when the sheets S1, S2 have shifted in the transport direction on the finishing plate 446, the sheets are transported just at a shifted position.